Professor James Economy's Group
Liquid Crystalline Polyesters
Annealing LC Copolyesters
Thermotropic liquid crystalline (LC) copolyesters have found a wide range of high tempreature applications such as electrical connectors, high temperature adhesives, and protective coatings. Critical to the successful implementation of the above technologies is the need for a fumdamental understanding of the microstructure change by high temperature annealing. Based on our prefvios work we briefly describe some of our earlier work elucidating the nature of high temperature annealing in LC copolyesters. The following discusses three major areas that we feel will elucidate the nature of LC copolyester microstructure.
Several years ago Economy et al. took an initial step to certify the randomization process of HBA/HNA copolyesters using 13C NMR. When a mixture of homopolymers was compression molded at 450°C, within 10 seconds, the NMR spectrum of the sample displayed four carboxyl resonances which corresponds to the four dyad arrangements. These results can be interpreted as the first direct evidence for randomization by interchain transesterification reactions (ITR, see Polyester Thermosetting Resins). These reactions are very rapid; with 1,000 bond swappings happening every 10 seconds. Because of this rapid character we have recently shown that homogeneously strong, high temperature, adhesive bonds can be made between HBA/HNA copolyester films on metal substrates.
Annealing LC copolyesters near the transition temperature dramatically increases the transition temperature via ordering processes. A change in microstructure from hexagonal to orthorhombic, an increase in density, and a decrease in solubility are key indications of an increase in ordering. Schneggenburger et al. reported on a change of sequence distribution by ordering processes. When a random copolyester is annealed near its transition temperature, 13C NMR results show a shift in the BN:BB diad peak ratio indicating a change in sequence distribution by ITR.
An Unusual Degradation Process
An unexpected degradation process was observed in step ordering of the HBA/HNA copolyesters. The fact that this degradation process occurs only in HBA/HNA copolyesters suggest that the naphthalene unit may be more prone to these kinds of rections. Stepwise annealing of the HBA/HNA copolyesters resulted in a complete loss of crystallinity. These degredation reactions occur only within the highly ordered crystalline orthorhombic structures. The characteristics of the degradation of HBA/HNA copolyesters include reduced flow properties, decreased solubility, discoloration, and thermal irreversible properties. Currently we are researching the rheological properties to understand the change of molecular structure by step order annealing processes.